In the case of the grinding system in open-circuit operation, the grinding stock is fed once through the grinding plant and then has the desired product properties with respect to fineness and grain distribution. It would be possible to achieve a marked increase in the efficiency of the grinding system by using classifiers, wherein the grinding stock is passed through a mill-classifier circuit. In this context, the fresh material can be fed in upstream or downstream of the mill. Overgrinding, as in open-circuit operation, is reduced and the product properties can be set better. The mills used in cement and mineral production are in particular ball mills, vertical and horizontal roller mills, attritors and high-pressure roll presses. The throughput power of these mills is, however, limited and approximately constant independent of the separation result in the classifier. Therefore, when using these mills, the quantity of finished product per hour can be increased only by means of an improvement in classification.
Improved classification means that there is less fine material (finished product) in the coarse material of the classifier and, accordingly, the fraction recirculated to the mill is reduced, such that an accordingly greater fraction of fresh material can be processed in the mill.
In the past, classifiers developed from a static to a dynamic air stream classification. That means that the air flow and gravity were supplemented by rotation. The conventional construction involves an air-guiding system being arranged around a rotating rotary cage, such that a classifying zone forms between the rotary cage and the air-guiding system. The feedstock to be classified is introduced either with the classifying air stream through the air-guiding system and/or directly into the classifying zone from above. In order to achieve as homogeneous as possible a flow in the classifying zone, a great variety of air-guiding systems and rotary cage shapes have been developed.
Since, in cement and mineral production, the material to be classified has a tendency to agglomerate, further measures have been used, in order to increase efficiency, which deagglomerate and/or better distribute the grinding stock within the classifier. To that end, for example in the case of embodiments in which grinding stock is supplied via the rotor cover, so-called impact strips have been provided, which at least partially break up the agglomerates. In DE 100 40 293 A1, the air-guiding system is formed by conical trickle baffles, such that the grinding stock supplied from above at least partially deagglomerates through contact with the conical trickle baffles. A further measure for improving classification efficiency involves increasing the circumferential speeds at the rotary cage by means of higher rotational speeds. However, this can be brought about only in conjunction with a higher flow speed, in order to retain the product properties (in particular fineness). Although this coupling leads to an improvement in classification efficiency, higher rotor rotational speeds and higher volume flow rates are necessary.